Third-order Cycles Research Articles

Lithofacies characteristics and genetic mechanism of hybrid sedimentary rocks in Da'anzhai member, Sichuan Basin

The Jurassic Da'anzhai Member in the Sichuan Basin is a stratigraphic unit characterized by widespread mudstone and hybrid sedimentation. It comprises various lithofacies types and sedimentary structures, including massive structure, laminated structure, mud rip-up clasts and erosion surfaces. By means of detailed analysis of core, logging, thin sections, scanning electron microscope, XRD and XRF data, six lithofacies types are identified in the Da'anzhai Member, and the genetic mechanisms of these lithofacies such as sandy debris flow, turbidity current, bioclastic flow and still water sedimentation are analyzed based on the characteristics of lithofacies. A complete third-order cycle was identified in the LA 1 and RA 1 well based on the variation in the value of gamma ray, bioclastic content as well as lithofacies characteristics. The paleo-sedimentary environment of Da'anzhai Member of Sichuan Basin is reconstructed on the basis of geochemical proxies such as Sr/Cu, Ca/(Ca + Fe), V/(V + Ni), V/Cr, Ti, Al, Al/(Al + Fe) and Fe/Mn. Different sedimentary environments in the Da'anzhai Member controlled the characteristics of lithofacies distribution and the lake level (water depth) primarily controlled the scale and frequency of the development of hybrid sedimentation and mudstone. During the early and late sedimentary periods of the Da'anzhai Member, the climate was arid, water salinity was high, reducing conditions were weak, and the water body was deep. Sediments such as sandy and bioclastic materials in shallow areas were transported to deeper areas through gravity flows under the influence of sudden events like earthquakes, floods or storms and interbedded with still water mudstones. In the middle period of the Da'anzhai Member, the climate was humid, water salinity was low, reducing conditions were strong, and the water body remained deep. A large amount of deep-water mudstone developed in this period, with gravity flow deposits appearing as thin layers. Finally, the comprehensive sedimentary model of the study area is completed based on this characteristics.

Characteristics and Geological Significance of High-Frequency Cycles in Salinized Lake Basins: The Paleogene Kumugeliemu Group in the Xinhe Area, Northern Tarim Basin

Salinized lake basins have distinctive sedimentary response characteristics, similar to marine shallow-water carbonate platforms. High-frequency cycles can also be used to reveal more sedimentological information, such as relative lake-level fluctuations, lithofacies sequence combinations, and paleogeographic evolution. In this article, a comprehensive study on the stratigraphic shelf delineation and high-frequency cycles of the Paleozoic Kumugeliemu Group in Xinhe area, northern Tarim Basin, was performed using drilling cores, logging curves, and seismic analyses. As a result of the study, the following data were obtained: the three sets of marker beds in the Kumugeliemu Group in the study area could be divided into a bottom sandstone component (E1-2 km1), a lower gypsum mudstone component (E1-2 km2), a salt rock component (E1-2 km3), and an upper gypsum mudstone component (E1-2 km4) by petrology vertical overlay combination and isochronous tracking correlation, which constituted two third-order cycles (ESQ1, ESQ2). They were further divided into seven fourth-order cycles (Esq1–Esq7). Due to the droughty and saline lacustrine depositional system background, the internal rock fabric changed frequently and showed a periodic vertical overlay pattern. Stratified gypsum salt, gypsum mud (sand) rock, and gypsum rock were used as the cycle interface. A single cycle was mainly characterized by an upward shallower depositional sequence of rapid lake transgression followed by a slow lake regression, composed of massive sandstone–lamellar mudstone–lime dolomite–gypsum rock, massive sandstone–lamellar mudstone–gypsum rock (gypsum salt), massive sandstone–massive gypsum mud (sand) rock–gypsum rock, and other cycle structure types. The complete sedimentary cycle was superposed by a single cycle and compared by the inter-well thickness difference, indicating that the study area had a paleogeomorphology pattern of “West-Low–East-High”. The thickness of the cycles decreased gradually from bottom to top vertically, and five sedimentary stages were determined, i.e., freshwater, brackish, brackish water, salt lake, and semi-saltwater, reflecting the evolutionary process of increasing salinity, lake basin filling, and gradual salinization and shrinkage.

The high-frequency sea-level change in the aftermath of the Marinoan snowball Earth: Evidence from the Doushantuo formation in the northern margin of the Yangtze Craton, South China

The rapid rise of glacioeustatic change is the most extreme paleoenvironment alteration in the aftermath of Snowball Earth. Although geologists conducted a lot of multi-subdiscipline research on this issue previously, there still exists the potential room for further discussions of the process in detail. For decades, the practice proved that the Fischer plot is a simple and robust tool to illustrate the fluctuations of accommodation patterns v.s. cycle sets or strata depth which could be interpreted as relative sea-level changes. This research simulates the Fischer plot to unravel the sea-level change in the aftermath of Marinoan glaciation by measuring the lower Ediacaran Doushantuo Formation in Shennongjia, South China. The result shows that 131 fifth-order cycles and nine third-order cycles help us propose a completed second-order cycle variation of ice melting-forced sea-level change, i.e., (1) early high-frequency and slow to rapid stepwise rising, and (2) followed by a stable decrease in the latter. In addition, the vertical sedimentary facies of the lower Doushantuo Formation display, in ascending order, (1) intertidal carbonate rock, (2) subtidal clastics with turbidite, and (3) intertidal lagoon fine clastics, indicating the process of the relative waxing and waning of sea-level. Such interpretation of the Fischer plot and the sedimentary facies’ vertical evolution is beneficial for studying the high-frequency sea-level change and paleogeographic reconstruction of post-Marinoan deglaciation.

COMPARISON OF THE GLOBAL ONLAP CURVE AND SEA-LEVEL CHANGES WITH TIMING OF UNCONFORMABLE SEQUENCE BOUNDARIES FORMATION OF A PALEOCENE SUCCESSION, SIRTE BASIN, LIBYA

The Paleocene sedimentary successions in the NW Sirte Basin in north central Libya have complex stratigraphic architecture and internal configuration. This stratigraphic architecture can provide information on timing of sea-level changes and associated onlaps. Therefore, this study uses the formation timing of the sequence boundaries separating between the stratigraphic sequences and correlate it with the global online curve and global sea-level changes. The number of sequences recognized in the Paleocene succession of the studied platform matches the number of third-order cycles, indicating that each sequence represents a single thirdorder cycle and was created as a consequence of one thirdorder cycle of sea-level fluctuation.

Sequence Stratigraphy and Sedimentary Environment of the Shiranish Formation, Duhok region, Northern Iraq

The Shiranish Formation is cropped out in several areas in northern Iraq. A stratigraphic and facies study was conducted within the Duhok region to determine the sedimentary environment. Three microfacies, reflecting the various subenvironments within different shelf parts of the deep sea, have been identified within the Shiranish Formation. Four depositional environments are identified: slope, the toe of slope, deep shelf, and deep-sea or cratonic deep basin. The Shiranish Formation in the Duhok region, Northern Iraq, was deposited in an open shelf carbonate platform. The Shiranish Formation sequence is divided into six third-order cycles in the study area. These asymmetrical cycles reflect an imbalance between the relative level of the sea and the production of carbonate, and each one reflects a rise in the sea level following a period of standstill. There is a two-sequence boundary type SB-2 that defines the surface. The Shiranish sequence developed in a high-subsidence area that played the main role in the evolution of the formation. It was deposited on a carbonate platform with high subsidence due to major transgression, wherever the successive sea-level rise and stillstand episodes persist.

Paleoenvironmental reconstruction, paleoecology and sequence stratigraphy of some Nummulites buildups in Egypt

The present study deals with microfacies analysis, paleoecology and sequence stratigraphy of the Lower–Middle Eocene rocks in central Nile Valley and west-central Sinai. Four rock units were marked (Minia, Darat, Samalut and Maghagha formations). Eight facies belts were interpreted including lagoon, back bank, main bank, fore bank and open marine outer bank facies and deposited on a gently dipping platform ramp. This ramp might be further divided to three main faunal associations depending on the depth of the water and associated fauna. Four major depositional sequences (DS1–DS4), reflecting third-order cycles and separated by two prominent sequence boundaries (SB1 and SB2) were marked. The record of these sequences reflects the changes in eustatic sea level plus the regional tectonism in the area. Every sequence includes facies associations, which constitute lowstand and/or transgressive and highstand systems tracts. Based on faunal elements and facies analysis, the water salinity, water temperature, nutrients, substrate, water depth and hydrodynamic behavior were discussed. The fine-grain size of the deposits, the loss of good connectedness and the occluding of the Nummulite tests by other minerals could give rise to low reservoir potential. The sedimentation was affected by the tectonic activity of the Syrian orogeny. Moreover, both external and internal mechanisms have controlled the carbonate sedimentation, basin construction and sequence bounding surfaces.

Depositional environment and sequence stratigraphy architecture of continuous Upper Permian and Lowermost Triassic deep marine deposits in NW and SW Iran

The end-Permian mass extinction, the greatest biotic crisis through the Phanerozoic, caused a severe loss of marine organisms in terms of abundance and diversity. Knowing sea level fluctuations, environmental conditions and the sequence stratigraphic architecture of the Upper Permian deposits will enable us to reconstruct events before the mass extinction and across the Permian/Triassic boundary in a regional and global context. The Upper Permian deposits are examined in well-known sections in Julfa, NW Iran and Abadeh in SW Iran using microfacies analyses, depositional environment interpretation and sequence stratigraphic framework. Detailed petrographic examination and microfacies analyses show third- and fourth-order cycles both in Wuchiapingian and Changhsingian deep-water distally deposited strata of Julfa and Abadeh sections. Considering Iranian tectonics during Permian through Triassic time, there was rifting accompanied by drift and subsequent subsidence. Tectonics was a main factor that influenced the depositional environment and sequence stratigraphic framework in the study areas. According to previous studies, since subsidence increased in a North to SW direction during Middle Permian through Triassic time, this effect is seen in lithologic characteristics of the Wuchiapingian with more mid-ramp settings in Julfa Beds in Julfa area, compared to the more outer-ramp settings of the Unit 6 of the Hambast Formation in Abadeh area. Tectonic subsidence influence was also responsible for creating the fourth-order cycles within third-order cycles despite the absence of common glacial events to create high-frequency cycles in Iran during the Lopingian. The Lopingian successive deepening in Iran caused the faunal changes which is evident in decreasing trend of body size and biodiversity in brachiopods and ammonoids and also marks the beginning of the transgressive system tracts during this time interval.

Paleoenvironment, sequence stratigraphy and source rock potentiality of the Cenomanian-Turonian boundary sediments of Southern Tethys

The Abu Qada Formation is a reference interval for the Cenomanian–Turonian boundary event in the northeastern margin of Africa. In this paper, geochemical, biostratigraphical, and carbon isotope ratio studies are presented using high-resolution well data yielded from the Gulf of Suez, Egypt to evaluate the paleoenvironmental condition, sequence stratigraphy and source rock potential of the Upper Cenomanian/Lower Turonian Abu Qada Formation. A total of 12 core samples and 4 ditch cuttings were collected from OCT-A8St1 well in the October Oil Field for geochemical, palynological and calcareous nannoplankton analyses. Cyclostratigraphic analysis was prepared to characterize the electrofacies change along the Cenomanian/Turonian boundary interval including the Oceanic Anoxic Event 2 (OAE2). The Predictive Error Filter Analysis (PEFA) patterns and Integrated Predictive Error Filter Analysis (INPEFA) curves found 3rd order cycle of the OAE2 within the global third-order cycle Ce5. A stable carbonate isotope analyses with total organic carbon (TOC) resulted in a remarkable positive excursion δ13Ccarb (3.67‰) associated with change in the TOC content. The quantitative analysis of calcareous nannofossils indicates the high productiveness of nannoplankton around the Cenomanian/Turonian boundary interval. The high abundance of the Discorhabdus genus proves the paleoenvironmental change during the OAE2 interval. The palynofacies interpretation indicates the lower part of Abu Qada Formation to be deposited in a slightly reducing conditions at the inner neritic setting, while the upper part of Abu Qada to lower part of Wata formations were deposited in deeper marine setting under anoxic conditions. These changes in the overall depositional conditions are documented by the remarkable change in palynofacies and organic carbon content (TOC). Dark gray shale and carbonate sediments are interpreted to be a good source rock potential by kerogen quantified palynofacies analysis indicating oil and gas prone producing materials (Kerogen types II and II/III). The best source rock quality is associated with the Abu Qada Formation's OAE2 record. These criteria allow us to identify the occurrence of OAE2 record in the Gulf of Suez petroleum province as a useful tool for high-resolution chronostratigraphic analysis.

Third-Order Microbialitic Cycles of Kuwait Formation: A Gateway to Early Oligocene Orbital Obliquity

Third-Order Microbialitic Cycles of Kuwait Formation: A Gateway to Early Oligocene Orbital Obliquity

Sequence Stratigraphy of the Fatha Formation in Shaqlawa Area, Northern Iraq

A surface section of the Fatha Formation (Middle Miocene) was studied in the Shaqlawa area, Erbil, Northern Iraq. It consists of siliciclastic silt, evaporates, and carbonates in a mixed siliciclastic silt composition. The Fatha Formation in the study area can be divided into two members of variable thickness based on rocky differences. Depositional settings ranged from shallow open-marine and restricted-hypersaline to supratidal and continental (sabkha, fluvio-deltaic, and exposure). It is bounded below by a type one sequence boundary above the Eocene Pila Spi Formation and marked by conglomerates. The upper sequence boundary with the Injana Formation is conformable. Thirteen sedimentary facies were distinguished in the Fatha Formation within the Shaqlawa region of northern Iraq and include sandstone to mudstone, wavy bedded sandstone to mudstone, Flaser bedded sandstone to mudstone, Marl, sandstone, cross lamination sandstone, Trough cross bedded sandstone, Planar cross bedded sandstone, marly limestone lithofacies, bioclastic grainstone to packstone microfacies, bioclastic lime mudstone to wackestone microfacies, lime mudstone-wackestone microfacies, and gypsum lithofacies. The depositional environment of the formation was inferred based on the facies association concepts. The succession formation can be divided into several third-order cycles, which reflect fluctuations in the relative sea-level rise. High-frequency cycles of transgressive System Tract and Highstand System tract. Fundamental to the evolution of the sequence, in this case, is the local tectonic component.

Seismic Stratigraphic Analysis and Hydrocarbon Potential of Miocene-Quaternary Deposits in the Western Nile Delta Basin

This study focuses on the subsurface Miocene-Pleistocene sedimentary sequence of the Western Delta of Deep-Sea field. Analysis of seismic, based on obtainable well data, and seismic data, allows us to divide the studied successions into two mega sequences: Pre and Post Messinian complexes resulting in transgressive-regressive sedimentation cycles of sea level during the evolution of the Miocene-Pleistocene subsurface sedimentary sequence. The Relative level of the sea was extremely falling in the time of the Messinian period, although it was largely rising at the time of the lower to Middle Pliocene. Pre-Messinian complex encompasses the Miocene strata, while the Post-Messinian complex consists of a thickness pattern of deposits in the time of Pliocene to Pleistocene and ended up with Holocene. The comprehensive study presented here divides these complexes into several orders of sea-level cycles. Pre and Post-Messinian complexes are consisting of several third-order cycles, which is called a depositional sequence, hence the thickness pattern starts from Sidi Salem Formation and ends up with Mit Ghamr Formation (Pleistocene). The interpreted anticline represents a characteristic overlap that can create an appropriate structural trap for hydrocarbons in the sandy intermission of the formations of the Western Deep-Sea Delta field such as Kafr El Sheik siliceous clastic. Besides, the recognized individual and various hidden routes, such as channel and sub-channel in the Pre-Messinian complex are approved for additional inspection to discover hydrocarbons.

Subsurface geologic imaging of northeastern Tunisia during the Middle to the Upper Eocene: Insights from integrated geophysical interpretation

The Middle to Upper Eocene series are characterized by multiple hiatuses related to erosion, nondeposition, or condensed series in the Cap Bon and Gulf of Hammamet provinces. We have performed an integrated study taking advantage of surface and subsurface geology, faunal content, borehole logs, electrical well logs, vertical seismic profiles, and surface seismic sections. Calibrated seismic profiles together with borehole data analysis reveal unconformities with deep erosion, pinch outs, normal faulting, and basin inversion that are dated Campanian, intra-Lutetian, and Priabonian compressive phases; these events were also described at the regional scale in Tunisia. Tectonics, sea-level fluctuations, and climate changes closely controlled the depositional process during the Middle to Upper Eocene time. The depositional environment ranges from internal to outer platform separated by an inherited paleo high. We determine eight third-order sequences characterizing the interaction among tectonic pulsations, sea-level changes, and the developed accommodation space within the Middle to Upper Eocene interval. We correlate the obtained results of the Cap Bon-Gulf of Hammamet provinces with the published global charts of sea-level changes, and we find a good correspondence across third-order cycles. Model-based 3D inversion proved to be a solution to model the lateral and vertical lithologic distribution of the Middle to Upper Eocene series.

Climate-driven palaeofloral fluctuations on a volcanic slope from the low latitudes of the Palaeotethys (early Permian, West Sumatra)

The present paper focuses on plant diversity fluctuations within three ecosystems from a volcanic slope in the sequence analytical context of three superimposed cyclicities. This analysis was possible thanks to the isotopic age evaluation of the volcanic Merangin section that bracketed the Mengkarang Formation on West Sumatra between 296.77 ± 0.04 and 296.14 ± 0.09 Ma (Asselian, Lower Permian). The Merangin section consists of a 490 m long fining and thinning tuffaceous aggregational sequence composed of eight short-eccentricity fining and thinning upwards parasequences. The palaeoflora along the sequence varies from tropical wet taxa, comprising cordaitalean and ferns, to mesic-xeric taxa, typified by seed ferns and early conifers. Herein the palaeofloral content of 14 consecutive lahars from a single volcanic accretion wedge in the seventh parasequence is described in detail. It displays a gradual increase in the ratios of (seed) ferns (Macralethopteris hallei, Sphenopteris sp. and gigantopterids) and early conifers (Dicranophyllum molle, Tobleria bicuspis), reaching a 100% in the 6th bed, and returning to 100% cordaitalean and ferns. The high ratio of seed ferns and early conifers in the 6th lahar is interpreted as reflecting a temporary lowering of the tree line caused by a glacial maximum from a high order cycle (obliquity or precession), superimposed on a short-eccentricity cycle, which in turn is superimposed on our third-order cycle palaeofloral transition described earlier for the whole sequence. Along this lower Permian section from the low latitudes from the Palaeotethys, glacial-interglacial cycles are seen to affect the fluctuations of the vegetation line in a chiefly similar way as during the Quaternary. The diversity fluctuations for three ecosystems (wetlands, lower and upper slope) are modelled for two of the three or more superimposed cycles. Emphasis is put on the appearance of a glacial palaeoflora on top of the sequence.

Sequence-stratigraphic dynamics: Variations of genetic stratigraphic units driven by basin subsidence

Rapid basin subsidence can cause the stacking of all strata deposited within a third-order eustatic cycle in a backstepping pattern. However, such extreme changes in stratal stacking patterns remain unexplored due to the lack of case studies. In this study, we first introduce the concept of sequence-stratigraphic dynamics based on previous achievements, and investigate the normal variations of genetic stratigraphic units of a third-order cycle driven by basin subsidence using mathematical simulations and a case study. We have established a dynamic equation and a system for the evaluation of the variations in genetic stratigraphic units based on which 6 potential driving mechanisms and 36 genetic unit variations, including 2 extreme variants of normal and reverse, were identified. The results confirm that third-order cyclic sea level change (Rsea-level), generalized basin subsidence (Rbase), and the sediment supply (Rsediment) drive the changes in the stratigraphic architecture. However, the variations of genetic stratigraphic unit are driven by the residual basin subsidence rate (Rbase - Rsediment). The stratal stacking pattern depends on the ratio of sediment supply rate to the residual accommodation rate (Rsea-level + Rbase - Rsediment). Sequence-stratigraphic dynamics are applicable to all basins, and may lend themselves as a theoretical basis for wider stratigraphic correlations.

Calculating the Eroded Thickness Corresponding to a Short-Term Tectonic Uplift with Milankovitch Theory: A Case Study of the Middle Permian Maokou Formation in Southeastern Sichuan Basin, Southwest China.

The Middle Permian Maokou Formation in the southeastern Sichuan Basin is a typical carbonate karst reservoir. At the end of the Middle Permian, a short-term tectonic uplift (Tungwu movement) occurred in the upper Yangtze region, causing the formation of dissolved fissures and holes. To determine the location of the high-quality reservoir, this paper calculated the eroded thickness using the Milankovitch theory. Based on the gamma logging data of the six wells in the southeastern Sichuan Basin, the dominant frequency and the astronomical time scale were evaluated via frequency spectrum analysis, continuous wavelet transform, and empirical mode decomposition. In addition, we analyzed the relationship between Fischer curve characteristics and the variation of lithology. Last, four methods were used to calculate the eroded thickness, and the rationality was analyzed. Consequently, we identified four levels of Milankovitch cycles, i.e., middle eccentricity (e2), short eccentricity (e3), long obliquity (o1), and short obliquity (o2). Also, the Fischer curves of the six wells were divided into two forms related to local structural uplift. The residual strata of the Maokou Formation comprised three complete third-order cycles, and the boundaries were the 15th, 34th, and 54th e3 cycles. The deposition rate of bioclastic limestone was the lowest (2.12–5.36 cm/ka with an average of 3.30 cm/ka), whereas the deposition rate of argillaceous limestone was the largest (2.27–5.25 cm/ka with an average of 4.09 cm/ka). Among the four methods, the missing formation deposition rate method exhibited the most precise calculation results, while that of the seismic data method was relatively low. Generally, the eroded thickness of the Maokou Formation in southeastern Sichuan was in the range of 0–140 m, i.e., the eroded thickness in the west and south of X14 was relatively large (>100 m), while the area north of LS1 experienced the weakest denudation (eroded thickness < 40 m).

Spatiotemporal evolution of single sandbodies controlled by allocyclicity and autocyclicity in the shallow-water braided river delta front of an open lacustrine basin

Abstract The spatial and temporal evolution of the sandbody architecture of shallow-water deltas in open lacustrine basins is controlled by the classification of allocyclicity and autocyclicity. On the southwestern margin of the Ordos Basin, a braided river system deposited a shallow-water delta in the Late Triassic Period. Based on the principle of sequence stratigraphy and the hierarchical analysis of reservoir architecture, the spatial and temporal evolution of individual sandbodies in the Chang 81 member of the Yanchang Formation in the Zhenbei Oilfield is interpreted by utilizing data from cores, wells and outcrops. The research ideas are as follows: large deposition scale architectural elements (first- to third-order cycles, as defined by Miall) of different sequence levels are affected by allocyclicity associated with changes in tectonic activity, provenance, and sea level, and small deposition scale architectural elements (fourth- to fifth-order cycles, as defined by Miall) of different sedimentary facies mainly consist of individual sandbodies that are affected by autocyclicity associated with lake-level changes caused by various river processes. Based on previous studies, the results are as follows. The sedimentary characteristics of shallow-water deltas have been verified by core and outcrop data. In addition, three ultrashort-term cycles are identified on the basis of boundary sequences and lithofacies’ sequences in the outcrop section of the Rui River, and three sedimentary evolution stages of the delta front are defined. Finally, according to well data, five types of architectural elements at the level of single sandbodies are identified. The vertical superimposition and lateral contact relationships of different architectural elements indicate that during the three sedimentary evolution stages, the hydrodynamics weakened, strengthened slightly, and finally weakened substantially. Among the 20 kinds of architectural element spatial combination patterns formed by single sandbodies, primary and secondary sandbodies have great potential for hosting remaining oil. In the process of architectural spatiotemporal evolution, the geometry and connectivity of the underwater distributary channel gradually weakened, and the spatial relationship between the underwater distributary channel and other architectural elements increased. This article proposes a new method for researching shallow-water deltas and has some guiding significance for exploiting the remaining oil in oil fields.

Application of stable isotopes, trace elements and spectral gamma-ray log in resolving high-frequency stratigraphic sequences of a mixed carbonate-siliciclastic reservoirs

Integrated methods have been applied for the high resolution sequence stratigraphic analysis of a mixed petroliferous carbonate-siliciclastic succession of the Asmari Formation (Oligocene-Miocene) in the Zagros Basin. Detailed sedimentological (core description) and petrographical (one thousand one hundred fifty thin sections) analysis led to identification of more than 38 carbonate and siliciclastic facies (basin wide) that have been deposited in four types of depositional environments, including inner ramp, mid ramp, outer ramp and basin, in homoclinal ramp type carbonate platform. An integrated multidisciplinary approach including core descriptions, petrography, biostratigraphy, gamma-ray spectrometry (e.g., uranium (U), thorium (Th) and Potassium (K)), oxygen and carbon isotopes (Chemo-stratigraphy) and trace element analysis have been used for stratigraphic interpretations of this succession. Based on petrography, biostratigraphy, lateral and vertical facies changes, and GR log, 6 third-order cycles distinguished, whose boundaries correspond to distinct δ13C and δ18O negative peaks. Trace elements (Fe and Mn) contents along with oxygen and carbon isotopes variations are compared with distribution of sedimentary facies and depositional sequences to investigate and correlate stratigraphic boundaries, which are determined by sequence stratigraphy (GR log and facies analysis) and stable isotope stratigraphy (Chemo-stratigraphic) in shallow marine carbonate platform of the Asmari. Negative isotope and elemental peaks coinciding with the sequence boundaries while the positive isotope and elemental peaks are correlated with maximum flooding surfaces. Changes in the frequency of uranium (U), thorium (Th), and Potassium (K) has been used to investigate the stratal surface and sequence boundaries as well as oxidation and reduction states. Fourth-order sequences that formed during the formation of the third-order sequence could be distinguished and separated by uranium log.

Deciphering the upper ordovician Wufeng siliceous shale depositional environments (Wuxi, NE Chongqing) based on multi-proxy record

Abstract Based on stratigraphic variations of petrology, geochemistry (major, trace elements), and pyrite framboids, we study the sedimentary environment of siliceous rocks of the Wufeng Formation in Wuxi, Northeastern Chongqing, China. Coupled Al2O3/(Al2O3+Fe2O3), SiO2/Al2O3, with Al–Fe–Mn values indicate that Wufeng Formation are deposited in a continental margin and influenced by detrital input. UEF-MoEF and V/Cr–U/Th cross-plots suggest that the siliceous shale was formed in dysoxic to anoxic conditions. Redox proxies imply that the lower, the middle to upper, and the top part of the Wufeng sediments were deposited in an anoxic, dysoxic and oxic environment, respectively. Accordingly, the average size of pyrite framboids are gradually increased from bottom to top, showing that increased oxidisability. A complete vibratory third-order cycle of sea level fluctuations during the Wufeng deposition can be identified. During Wufeng to Longmaxi transition, the sedimentary environment exhibits a short-scale oscillatory pattern and was probably transformed from an outer shelf to inner shelf. Coupled multi-proxies are considered more reliable proxies for deciphering redox conditions in fine-grained sediment.

Different order cyclicity of the Paleozoic reef formation

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Sequence stratigraphic framework of the Maastrichtian shelfal to deep marine strata of Lower Indus Basin, Pakistan: implications for the petroleum potential

The present study integrates the facies and planktonic foraminiferal biostratigraphic data to construct a sequence stratigraphic model of the Mughal Kot Formation of Maastrichtian age in the Lower Indus Basin, Pakistan. The Mughal Kot Formation is comprised of 1000-m-thick sequence of hemipelagic sediments, interrupted by random influxes of coarse-grained sandy turbidites. The petrographic details revealed six sub-microfacies, indicating deposition in the middle shelf, slope and deep marine basin. The sequence stratigraphic model is based on the integration of biostratigraphic age and local depositional signals in the study area. The studied sedimentary succession is divided into one second-order and two third-order cycles while ten parasequences are identified within the complete cycles of transgressive, highstand and lowstand systems tracts. The depositional trends within the relative sea level curve of the Maastrichtian strata of the study area and the global sea level charts are quite dissimilar, which suggest a sustained transgression with pulsated tectonics in Lower Indus Basin. The integrated results of plug porosity/permiability and petrography reveal poor reservoir potential for the sandy turbidites and pelagic carbonates of the Mughal Kot Formation.